At present, laser projectors are utilized for assembly of components on a work surface, such as the assembly of components of a truss as disclosed, for example, in my U.S. Pat. No. 5,646,859. At least four targets, typically retro-reflective targets, are fixed relative to the work surface and a laser projector periodically scans the targets to calibrate the relative position between the work surface and the laser projector. This is presently achieved by measuring the target coordinates relative to the coordinate system of the tool and then locating the position of the projector relative to the tool by a process of resection (calculating the position of the projector where the known laser rays to the target pass through the known 3-D target coordinates). Once the relative position of a laser projector is established, the required deflection angle of the laser projector scanning minors can be calculated to trace the laser spot through the desired template pattern on the known three dimensional profile of the tool or work surface. The template pattern is projected as a series of vector movements with the laser sequentially tracing through the features of the pattern. If the trace through the pattern features occurs with sufficient frequency, the pattern may be displayed without visible flicker artifacts.
The precise location of the retro-reflective calibration targets is established by scanning the laser spot in a grid pattern over the surface of the target. Locating the position of four or more targets allows the calibration of the relative projector position. In the traditional process, a host computer calculates the projector location and required scanning vector mirror movements to trace through the defined 3-D template pattern, and then transmits the resulting vector display list of two axis scanning mirrors' movements to an imbedded computer in the projector which continually retraces the desired scanning sequence. Periodically, the scanning sequence is stopped and a target is located to check for variations in the projected pattern location due to a change in the position of the projector relative to the target or tool surface or to compensate for other factors, such as drift due to temperature variations in the environment. If a variation is detected, the targets are relocated and a new scanning sequence is calculated and transmitted to the projector.
However, scanning the target positions with the projector must be performed sequentially and, at most, a few targets may be scanned per second. As a result, tests for projection drift can only be performed intermittently and correction of the projector pattern results is a noticeable interruption in the projected pattern. While this traditional method can compensate effectively for slow-changing environmental conditions or intermittent occurrences, such as deliberate repositioning of the target or tool surface, it cannot correct for more dynamic changes, such as vibration in a ceiling mounted laser projector as loading cranes are moved or as a building movement occurs in response to air pressure variations due to wind gusts or opening of a loading bay door.
The prior art does include proposals for compensating for movement of a projector relative to a target surface using a camera to acquire the pose of the system relative to a target's surface. Using a camera to perform the tracking of the projector has two important advantages, namely (i) modern cameras can operate at high speed, locating targets hundreds of times per second, and (ii) target detection is independent of projection and does not interfere with the dynamic motion of the projected image. Generally, however, the projector disclosed in the prior art is a video projector (i.e., displaying raster images at a fixed refresh rate, such as 30 or 60 frames per second). In contrast, the method of this invention corrects for a dynamically displayed vector image produced by a scanning laser. While the entire laser pattern may only be refreshed more slowly (e.g., 40 times per second), the individual points traced are updated more than 10,000 times per second as discussed further below. One exception is disclosed in published PCT Application WO 201213645 of Keitler, et al which states that the disclosed dynamic tracking method may be used for either video or laser projectors; however, the disclosed method relies upon unique configured targets which have not proven feasible commercially, particularly for laser projectors.